1. Field of the Invention
The present invention relates generally to ionospheric modeling, and in particular, to a method, apparatus, and article of manufacture for including ionospheric radio occultation data between satellites and global positioning system (GPS) transmitters to improve ionospheric modeling.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by names and years of publications (e.g., Author [2006]). A list of these different publications ordered according to these references can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Ionospheric remote sensing is in a rapid growth phase driven by an abundance of ground and space-based GPS receivers, new UV (ultraviolet) remote sensing satellites, and the advent of data assimilation techniques for space weather. The success of the GPS/MET (Global Positioning System/Meterology) experiment in 1995 inspired a number of follow-on radio occultation missions for profiling the atmosphere and ionosphere. These include the Argentine Satelite de Aplicanciones Cientificas-C (SAC-C), the U.S.-funded Ionospheric Occultation Experiment (IOX), and Germany's Challenging Minisatellite Payload (CHAMP) (Jakowski and Wilken [2006]). The joint U.S./Taiwan Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC—http://cosmicio.cosmic.ucar.edu/cdaac/index.html), a new constellation of six satellites, nominally provides up to 3000 ionospheric occultations per day. The COSMIC; 6-satellite constellation was launched in April 2006 and observed final orbits in November, 2007. COSMIC now provides an unprecedented global coverage of GPS occultation measurements (between 1400 and 2400 good soundings per day as of June 2009), each of which yields electron density information with 1 km vertical resolution. Calibrated measurements of ionospheric delay (total electron content or TEC) suitable for input into assimilation models are currently made available in near real-time (NRT) from COSMIC with a latency of 30 to 120 minutes. Similarly, NRT TEC data are available from two worldwide NRT networks of ground GPS receivers (75 5-minute sites and 125 hourly sites, operated by JPL [Jet Proplultion Laboratory] and others).
The combined ground and space-based GPS datasets provide new opportunities to more accurately specify the 3-dimensional ionospheric density with a time lag of only 15 to 120 minutes. With the addition of the vertically-resolved occultation data, the retrieved profile shapes represent the hour-to-hour ionospheric weather much more accurately (Komjathy et al. [2010]). The process has begun where COSMIC-derived TEC measurements are integrated with ground-based GPS TEC data and such data is assimilated into models such as the JPL/USC Global Assimilative Ionospheric Model (GAIM) (Hajj et al. [2004]; Hajj and Romans [1998]; Mandrake et al. [2005]) so that three-dimensional global electron density structures and ionospheric drivers can be estimated. Recently the COSMIC GPS measurements along with ground-based GPS measurements have been assimilated into JPL/USC GAIM for a study of ionospheric storm (Pi et al. [2009]) revealing distinguished features of equatorial anomaly enhancements.
Over the course of the past 15 years, the Global Ionospheric Mapping (GIM) software developed at the Jet Propulsion Laboratory (Mannucci et al. [1998]) has been used to compute high precision slant ionospheric delay by removing the satellite and receiver differential biases from ionospheric observables using ground-based GPS receivers.
In view of the above, what is needed is a method to estimate ionospheric observables that can be used (e.g., in combination with other systems) to estimate a global 3D electron density field.